For portable power equipment such as chain saws, mufflers have since long in some cases been equipped with so called spark arresters. This arrester was then often positioned at the outlet of the muffler with the function to trap any sparks so that these did not exit the muffler. For a two-stroke engine chain saw for instance, the exhaust gases have a temperature of approximately 600 degrees C. when entering such a spark arrester screen. The mesh is commonly made of stainless acid proof material in order to stand such high thermal loads.
Small combustion engines have since long had comparatively high exhaust emission levels. This particularly applies to two-stroke type engines, because the combustion is more incomplete than in the case of four-stroke engines. Increased demands concerning exhaust emissions from small engines have however resulted in the introduction of catalytic converters for different types of small engines. This also applies to small engines for portable equipment such as chain saws etc.. Several different solutions for so called catalyst mufflers, particularly for chain saws, have emerged. These mufflers are usually connected directly to the exhaust port of the cylinder. A catalytic element is placed inside the muffler, and in the element an exotherm chemical reaction takes place. For chain saws for instance, this implies a temperature increase inside the muffler from around 600 degrees C. before the catalytic element to around 1000 degrees C. after the catalytic element. The applicant does not know of any case where a muffler with a catalytic converter has been equipped with a spark arrester positioned in the exhaust flow downstream the catalyst element. Due to the high exhaust gas temperature after the element, the working conditions for such a spark arrester become very difficult. At the same time, it is in many cases preferable to position a spark arrester downstream the catalytic element in order to reduce risks for fire and accidents when using the power equipment.
The muffler with catalytic converter in accordance with the invention thus is essentially characterized in that a spark arrester, i.e. a structure that at least partly contains a wire mesh or a screen, usually made of metallic material, is positioned in the exhaust flow downstream a catalytic element. Furthermore the spark arrester screen is usually positioned in close proximity to the exhaust outlet. In order to withstand the high thermal loads on the spark arrester screen, so that a reasonable life time can be obtained, a number of different measures are used, on their own or in combination with each other. Partly design measures on the spark arrester screen itself to achieve a well distributed heating of the screen and good cooling, e.g. measures for separation of the exhaust flow. Partly restriction are use in the muffler upstream to achieve increased turbulence in the exhaust stream and thereby cause better distribution across the spark arrester screen and at the same time achieve better cooling of the exhaust stream before it reaches the spark arrester screen. These and other characteristics and advantages will be apparent from the ensuing description of the preferred embodiments and with the support of the drawing figures.